This black gizmo is on the passenger side of the engine compartment, just aft of the headlight.

What is it? What does it do?

At idle it vibrates and makes its own special rattle noise.

There are two horns in front of the radiator, so I'm pretty sure its not a horn.

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1st MBz: 1982 300SD
2nd MBz: 1987 300SDL
3rd MBz: 1995 S420
4th MBz: 1987 190DT
5th MBz: 1984 300SD w/1983 300DT engine
6th MBz: 1999 C230k I'm 3rd owner, got it w/57,235 miles.
and manages Mom's 2007 R320 CDI